In the study of psychology, idiographic describes the study of the individual, who is seen as an entity, with properties setting him/her apart from other individuals. Nomothetic is more the study of a cohort of individuals. Here the subject is seen as representing a class or population and their corresponding personality traits and behaviours. The terms idiographic and nomothetic were introduced to American psychology by G. Allport in 1937, who borrowed them from the German philosopher Wilhelm Windelbrandt.

Nomothetic and idiographic are terms coined by Kantian philosopher Wilhelm Windelband to describe two distinct approaches to knowledge, each one corresponding to a different intellectual tendency, and each one corresponding to a different branch of academe.

Nomothetic is based on what Kant described as a tendency to generalize, and is expressed in the natural sciences. It describes the effort to derive laws that explain objective phenomena.
Nomothetic literally means "proposition of the law" (Greek derivation) and is used in both philosophy (see also Nomothetic and idiographic) and in psychology with differing meanings. In psychology, nomothetic measures are contrasted to ipsative or idiothetic measures, where nomothetic measures are measures that can be taken directly by an outside observer, such as weight or how many times a particular behavior occurs, and ipsative measures are self-reports such as a rank-ordered list of preferences.

Idiographic is based on what Kant described as a tendency to specify, and is expressed in the humanities. It describes the effort to understand the meaning of contingent, accidental, and often subjective phenomena.

Usually, nomothetic approaches are quantitative, and idiographic approaches are qualitative, although the "Personal Questionnaire" developed by M.B. Shapiro, and its further developments (e.g. Discan scale) are both quantitative and idiographic.